1. Field of the Invention
The present invention relates to an automatic frequency control (AFC) circuit for a receiver, and more particularly to a controlling method and circuit for the voltage controlled oscillator within a receiver which utilizes the local oscillator comprising its quasi-synchronous detection circuit to generate the reference frequency for AFC frequency control.
2. Description of the Related Art
Conventionally, in order to realize automatic frequency control (AFC) of a radio receiver, a control voltage is determined based on frequency error information obtained from a carrier regeneration circuit or some other circuit of the demodulator, and the control voltage is used to control a reference oscillator (typically a voltage controlled oscillator (VCO) or similar circuit) of a quasi-synchronous detector.
FIG. 4 shows an exemplary construction of a conventional AFC circuit. Referring to FIG. 4, the AFC circuit shown is formed as an AFC control loop which includes a quasi-synchronous detection circuit 21, a low-pass filter 22, a demodulation circuit 23, a frequency control circuit 25 and a voltage controlled oscillator (VCO) 26.
In operation, the voltage controlled oscillator (VCO) 26 normally oscillates with a reference frequency roughly synchronized with the received signal inputted from an antenna amplifier (not shown) to the quasi-synchronous detection circuit 21, and the oscillation output of the voltage controlled oscillator 26 is inputted to the quasi-synchronous detection circuit 21. The quasi-synchronous detection circuit 21 performs quasi-synchronous detection using the output of the voltage controlled oscillator 26 as a reference frequency. The low-pass filter 22 removes unnecessary frequency components and noise components from the received signal outputted from the quasi-synchronous detection circuit 21. In the demodulation circuit 23, a multiplier 231 synchronously detects the output of the low-pass filter 22 using a regenerated carrier outputted from a carrier regeneration circuit 232 to regenerate the received data.
The carrier regeneration circuit 232 regenerates the received carrier from the synchronous detection output obtained from the multiplier 231 and outputs remaining frequency error information "a" of the received signal at the input of the demodulation circuit 23. The frequency control circuit 25 controls the control voltage for the VCO 26 based on the remaining frequency error information "a" so that the frequency error may be reduced.
Generally, an oscillator to be used as the frequency reference to a quasi-synchronous detector such as, for example, the voltage controlled oscillator (VCO) 26 in the AFC circuit of FIG. 4 is required to have a high frequency accuracy and a high frequency stability, because the output thereof is multiplied up to a frequency of the radio frequency band. This becomes critical because any frequency error of the oscillator which arises from an initial deviation, aging, a temperature characteristic or some other factor is amplified by the multiplication. As a result, as viewed from the demodulation circuit in the following stage, any frequency error originating from the VCO is added to the frequency error of the signal being received. This makes the performance requirements upon the demodulator more severe (in terms of frequency synchronization acquisition range).
In particular, if the VCO is reset for some reason, such as the turning on of the power supply to the apparatus or the changing over of a channel, then this gives rise to a problem of how to determine the initial set value for the VCO. In this instance, if the initial set value for the VCO after resetting is not close to the final converged value to be determined by AFC, then the frequency error arising from the frequency deviation of the VCO is superposed on the signal after detection, resulting in a degradation of the initial synchronization acquisition characteristic of the received signal.
Meanwhile, in a system of the type wherein the bandwidth of each channel and the channel frequency interval are narrow with respect to a radio frequency (such as a low data rate satellite communication system), there is the possibility that incorrect synchronization to an adjacent channel may occur. Further, where the deviation of the VCO is excessively large, there is a problem in that the frequency range of the signal after detection partially falls outside the pass band of the low pass filter 22, resulting in deterioration of the signal to noise ratio and hence in deterioration of the reception synchronization acquisition characteristic.
Where digital demodulation is employed, synchronization acquisition can be improved by adding, within the synchronization acquisition process of the received signal, a step of estimating the frequency error between the frequencies of the VCO and the received signal using Fourier transform calculation. This countermeasure, however, is disadvantageous in that the required time for synchronization acquisition is elongated by a time corresponding to the calculation time required for the estimation.
From the reasons described above, the conventional design requires the use of a high precision, highly stable (and consequently, expensive) oscillator which has low frequency fluctuation due to initial deviation, aging, or ambient temperature.